Conventionally, zinc oxide is known as a self-activated phosphor represented by ZnO:Zn and having a broad emission spectrum with a peak around 500 nm, and used as a green light emitting phosphor material excited by an electron beam for vacuum fluorescent displays. In this ZnO:Zn phosphor, the Zn-to-O ratio is greater than 1 and excess Zn is present. It is believed that the zinc interstitials or oxygen vacancies contribute to the green light emission of the phosphor.
On the other hand, zinc oxide is a compound semiconductor with a band gap of 3.4 eV, and is also strongly expected to be used, as a material capable of emitting light at shorter wavelengths in the ultraviolet region due to the interband transition, for phosphor exciting light sources, light emitting diodes (LEDs), etc. In fact, however, the above-mentioned zinc interstitials or oxygen vacancies are easily formed in zinc oxide, and conventional ZnO-based materials show only very weak emission in the ultraviolet region, although they exhibit intense emission in the green-blue region.
It has been reported that when zinc oxide is heat-treated in a hydrogen-containing gas atmosphere or hydrogen is introduced into zinc oxide by hydrogen plasma treatment or the like in order to solve this problem, ultraviolet emission is increased while green emission is suppressed (Patent Literatures 1 and 2).
The ultraviolet emission of zinc oxide is increased by the methods described in Patent Literatures 1 and 2, but it is still not enough.